Electrostatic Discharge (ESD) protection becomes challenging when the ESD protection window is very narrow. For instance in protecting the power train of a magnetic DC-DC converter, especially when hot plug-in specification is taken into account, the ESD protection is very narrow due to the high output power array associated with the converter. Ideally therefore the I-V characteristics of the ESD protection device should be substantially vertical. One type of device commonly used for ESD protection is a lateral PNP bipolar junction transistor (BJT), an example of which is shown in FIG. 1 FIG. 1 shows a prior art lateral PNP in cross-section. It includes a p-type emitter 100, a p-type base 102 formed underneath the p-type emitter 100 and extending laterally toward a p-type collector 104. The emitter 100 includes an emitter contact region 110, and the collector includes a collector contact region 114. The contact to the base, which is to the left of the emitter contact, is not shown in FIG. 1. The lateral PNP has the advantage that it provides high holding voltages and typically can handle relatively high currents. However, at values above those corresponding to the holding voltage, the current flowing through the device tends to saturate (typically in the milliamp range). In order increase the protection window and allow the PNP to operate at higher currents requires the size of the device to be increased, resulting in a large footprint.
One ESD protection solution with high current carrying capability is offered by SCR devices, which due to double injection of current can handle higher currents than PNP BJTs (approximately 10 mA/um compared to approximately 1 mA/um for PNPs). A cross section through a prior art SCR is shown in FIG. 2. Like the lateral PNP of FIG. 1, the SCR in FIG. 2 includes a p-type emitter 200 with emitter contact 210, an n-type base 202, which extends underneath the emitter 200 and continues laterally toward a p-type collector 204 with its collector contact 214. As shown in FIG. 2, the emitter contact 210 is connected to the base contact 212 in this case. However, the SCR structure differs from a lateral PNP in that it further includes an n-type region 206 (referred to herein as an n-emitter) with contact 216 on the low voltage side to define a pnpn structure with the emitter 200, base 202, and collector 204. This gives the SCR its double injection characteristics. However SCRs also have a down side caused by the double injection. They have a lower holding voltage than PNPs.
The present invention provides a new ESD protection structure that offers the high holding characteristics of a PNP device as well as the higher current handling capacity provided by an SCR.